Device-to-device (D2D) communication denotes a communication method in which two adjacent user equipments (UEs) exchange data with each other directly, not through a base station. In other words, the two UEs become, respectively, a source and a destination of the data, to perform communication.
As an example, D2D can be used by a local media server that provides a large amount of data (e.g., programs of a rock concert, and information on musicians) to participants at a rock concert, etc., or it can be used for the purpose of offloading for sharing a load of a base station, and so on.
Here, each UE connects to a serving cell to perform telephone communication, Internet access, etc., using an existing cellular link, and can directly exchange the aforementioned large amount of data with the local media server operating as a counterpart UE of D2D communication according to a D2D scheme. Meanwhile, a D2D link is established not only between UEs having the same cell as a serving cell, but also between UEs having different cells as serving cells.
In such direct communication between UEs, according to a D2D communication method based on a cellular network, a UE wanting to communicate with another UE requests link configuration from a central node (a base station in a cellular network) taking control, and the central node allocates radio resources for direct communication between the two UEs when the counterpart UE is close to the UE, such that direct communication can be performed between the UEs. Here, almost all operations of the UEs are managed by the central node, and it is possible to reuse radio resources allocated for a cellular link or another D2D link for D2D communication.
Existing cellular communication and the above-described D2D communication each have their merits and demerits. Thus, it is expected that a communication scheme that combines cellular mobile communication and the above-described D2D communication will ultimately be generalized. However, it has not yet been determined which one of a base station and a UE will take control of D2D communication combined with a cellular network. Centralized control by a base station and distributed control by a UE each have their merits and demerits.